Page 31 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
26 Geotechnical Engineering

oscillations confirm that paired rock strata on two sides of a rift are the same age,
an additional evidence for plate drift.

As shown at the right in Fig. 2.3, the Mid-Atlantic Ridge cuts through
Iceland, then runs southward like baseball stitching, parallel to the curves of
Africa and following the approximate middle of the Atlantic Ocean. The location
in Iceland is shown in Fig. 2.4 and explains volcanic activity and hot springs
in that island. The Pacific-Indian Ocean Ridge extends southward from the
San Andreas Fault through the Gulf of California, past Easter Island, westward
between Australia and Antarctica, and bends back north through the Red Sea.
Thus, if the marches continue, both the Gulf of California and the Red Sea will
become sites of volcanic activity and eventually enlarge to become oceans.

2.4.3 Three Kinds of Plate Bashing

The highest mountains in the world, the Himalayas, are being created by the
collision of two continental plates. At the other extreme, where two oceanic plates
collide they both plunge downward, forming a deep ocean trench flanked by a line
of volcanic islands or island arc. Examples are the Aleutians, Japan, Indonesia,
the Philippines, and the Solomon Islands.

Where a continental plate collides with a thinner oceanic plate, as along the
western edge of the Americas, the continental plate tends to override the oceanic

Figure 2.3
Plate margins,
volcanoes, and
earthquakes. At
the left the
boundary of the
Pacific
Plate defines the
‘‘Ring of Fire.’’
Zig-zagging of
faults was first
described by
H. Tuzo Wilson
of Canada. (U.S.
Geol. Survey.)

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